Round brush coordinates to 14 digits. Fixes rstudio#1634

Author: wch

srcjs/output_binding_image.js

@@ -1119,6 +1119,9 @@ imageutils.createBrush = function($el, opts, coordmap, expandPixels) {
     // For reversed scales, the min and max can be reversed, so use findBox
     // to ensure correct order.
     state.boundsData = coordmap.findBox(minData, maxData);
+    // Round to 14 significant digits to avoid spurious changes in FP values
+    // (#1634).
+    state.boundsData = mapValues(state.boundsData, val => roundSignif(val, 14));
 
     // We also need to attach the data bounds and panel as data attributes, so
     // that if the image is re-sent, we can grab the data bounds to create a new

srcjs/utils.js

@@ -56,6 +56,17 @@ function padZeros(n, digits) {
   return str;
 }
 
+// Round to a specified number of significant digits.
+function roundSignif(x, digits = 1) {
+  if (digits < 1)
+    throw "Significant digits must be at least 1.";
+
+  // This converts to a string and back to a number, which is inelegant, but
+  // is less prone to FP rounding error than an alternate method which used
+  // Math.round().
+  return parseFloat(x.toPrecision(digits));
+}
+
 // Take a string with format "YYYY-MM-DD" and return a Date object.
 // IE8 and QTWebKit don't support YYYY-MM-DD, but they support YYYY/MM/DD
 function parseDate(dateString) {